1. Field of the Invention
The present invention relates to an image forming apparatus which is capable of forming images in a double-sided mode.
2. Description of the Related Art
Conventionally, there have been image forming apparatuses such as copiers, printers, and facsimile machines, which have not only sheet feeding cassettes in which regular-size sheets are housed, but also, for example, a manual feed tray as a sheet feeding unit from which sheets of various irregular sizes (sizes that are not regular) can be fed. Some of such image forming apparatuses are able to detect a length of a sheet, which is placed on the manual feed tray, in a width direction (direction perpendicular to a conveying direction).
As described in U.S. Pat. No. 5,689,759, there is known an apparatus which, based on information on a sheet width obtained by width detection on a manual feed tray, displays prospective regular sizes on an operation unit and prompts a user to select a sheet size.
As described in Japanese Laid-Open Patent Publication (Kokai) No. H07-33G228, there is also known an apparatus which, based on an output from a sensor on a sheet conveying path in a case where a sheet with unknown size is conveyed from the manual feed tray, determines a sheet length of a sheet, which is fed from a manual feed tray, in a conveying direction and controls a bring-in gate of an intermediate tray in accordance with the sheet length.
In general, when double-sided image formation is to be carried out in an image forming apparatus, there may be sheets which can properly pass a double-sided path, on which a sheet with an image formed on a first side (front side) thereof is conveyed, and sheets which cannot properly pass the double-sided path. For example, when a sheet length in a conveying direction is shorter than a distance between sheet conveying rollers placed on the double-sided path, a sheet cannot be delivered between the sheet conveying rollers, and the sheet cannot pass the double-sided path. When such a sheet whose sheet length is too short is used, an automatic double-sided mode cannot be executed.
In an apparatus which has an inversion path for inverting a sheet when conveying it to a double-sided path, a sheet whose sheet length in a conveying direction is longer than a length of the inversion path cannot be inverted. Thus, when a sheet whose sheet length is too long, an automatic double-sided mode cannot be executed.
If the number of conveying rollers on a conveying path is increased to make distances between them short, or an inversion path is configured to be long in order to solve the problems described above, this will increase the cost and size of an image forming apparatus. In view of the circumstances, in many conventional models, an automatic double-sided mode is disabled in a non-specified size mode in which a sheet is fed from a manual feed tray with a sheet length in a conveying direction undermined.
However, even in a case where a sheet placed on a manual feed tray is a general regular-size sheet such as an A4- or B4-size sheet when image formation is to be performed in the non-specified size mode, usability will be significantly compromised if an automatic double-sided mode is disabled across the board. Moreover, in a case where a double-sided mode is enabled in the non-specified size mode in the apparatus described in Japanese Laid-Open Patent Publication (Kokai) No. H07-330228, when it is determined that a sheet has such a length that the sheet cannot be conveyed, there is no other choice but to dealt with this as a sheet jam, causing an increase in downtime of the image forming apparatus.